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SPE Library content related to rheology
High Speed Twin Screw Extrusion for Biodegradable Polymer Blends: Analysis of Compatibility and Rheology Prediction
This work examines the effects of high shear on the degradation and compatibility of blends of poly(propylene carbonate) and poly(butylene succinate) (PPC/PBS) in twin screw extrusion. Also, since solid PPC has poor flowing capabilities, different feeding methods for the TSE trials were compared for their ability to produce consistent results. The blends were compounded at 200, 500, 1000 and 2000 rpm. Viscosity measurements were used to estimate degradation, and it was found that the Maron - Pierce model for viscosity of composites accurately predicted blend viscosity at low shear. The viscosity change was inversely proportional to the screw speed, indicating matrix degradation. Moreover, the blend was more sensitive to thermomechanical degradation than the neat PBS. However, the molecular weight loss did not exceed 22% even at the highest screw speed of 2000 rpm. Finally, morphology investigation showed that the TSE blends had smaller droplet size with a broader shape distribution than the batch mixed blends. All results supported the idea that the high levels of shear stress are the governing factor in the morphology and the degradation of blends in twin screw extrusion.
Characterizing the Flow of Concentrated Slurries and Polymer Melts Using Percolation Theory Based Functions
The production of many polymers such as PPO is carried out as a two phase suspension in essentially a Newtonian carrier fluid. This paper brings together three percolation based theories that provide insight into the effect of fillers on the rheological response of concentrated Newtonian fluid slurries to shear rate. First, a previously proposed limiting, zero shear, viscosity model based on percolation theory concepts is reviewed. Second, all Newtonian fluid based slurries that have a high concentration of filler become shear thinning at some shear rate. A new theory is reviewed that correlates the power-law constant, n, to cluster formation of the fillers suspended in the fluid. Third, this cluster percolation based rheological analysis is then extended in this paper to a newly proposed model for the calculation of the ratio of infinite shear, ?8, to the zero shear viscosity, ?0, as a function of the power-law. Using literature data and a modification of this theoretical treatment, it is demonstrated that, ?8/?0 the viscosity ratio measurements correlate well with the power-law. Unfilled polymers also can reach the second Newtonian plateau and that has been seen to be related to the power law, n, of the polymer melt.
Analysis of Heat Transfer Coefficients and No-Flow Temperature in Simulation of Injection Molding
Material properties and boundary conditions are important inputs for any simulation. For the injection molding process, there are still many challenges to measure the polymer properties under processing conditions and there is not consensus about the thermal boundary conditions between the polymeric material and mold walls. This work is oriented to analyze the effect in the simulation results of the heat transfer coefficient (HTC), which is related to the boundary conditions, and the no-flow temperature (NFT), which is related to the material rheological behavior. The results for cavity pressure and temperature evolution from three well-known commercial software packages (CadMould®, MoldFlow® and Moldex 3D®) were analyzed and compared with experimental measurements. A semicrystalline material, PP 505P, from Sabic was used. In particular, the variation effect of heat transfer coefficients (HTC) and no-flow temperatures (NFT) were analyzed through a 32 factorial design of experiments (DoE). Based on the results, the most recommendable criteria to determinate NFT and HTC values for a semicrystalline material is proposed. The physical meanings of the obtained values are discussed.
Effect of Nucleating Agent and Two-Stage Expansion on the Morphology and Mechanical Properties of Microcellular Injection Molded Thermoplastic Polyurethane
Improved rebound resilience while lowering bulk density is desirable in several sport-wear applications. While rebound resilience properties generally deteriorate with reduction of the bulk density of the foam, a method of increasing rebound resilience by improving the control on microstructure is explored in this study. Low density thermoplastic polyurethane (TPU)/clay nanocomposite foamed parts were prepared using twin-screw extrusion compounding followed by microcellular injection molding. Samples with two densities were created by microcellular injection molding and an optional cavity expansion at a preset time during cooling. Scanning electron microscopy, rheological analysis, uniaxial compression tests, and rebound resilience tests were conducted on both of the non-expanded and expanded samples. Presence of well dispersed nanoclay in the TPU matrix acting as a nucleating and melt strengthening agent, coupled with cell growth at lower temperature helped achieve better microstructures, especially at high density reductions. The expansion of TPU at lower temperatures with directional second-stage expansion also helped to increase the rebound resilience, while achieving softer foams and lower hysteresis loss ratios at lower densities.
Modification of LLDPE by UV-Irradiation
Linear low density polyethylene (LLDPE) is used widely in applications like lamination and agricultural films, as well as a modifier for low density polyethylene (LDPE) and high density polyethylene (HDPE). The melt strength of LLDPE is modified in this study by introducing long chain branching (LCB) and/or crosslinking to its backbone through UV initiated radical reactions. Benzophenone (BP) is added as a photo-initiator to form free radicals and a UV lamp is used to irradiate solid sheets of the molded LLDPE resin, at different time intervals and intensity, according to a design of experiments (DOE). This paper aims to study the effect of photo-initiation on material properties, using linear-viscoelastic (LVE) rheological measurements, and differential scanning calorimetry (DSC).
Modified Polylactide with Improved Thermal and Rheological Properties for Foaming
Polylactide (PLA) is the most important bioplastic on the market due to its good mechanical properties and the permanent growth of the production capacity. One drawback of commercial polylactide is its too low melt strength and melt extensibility, which is disadvantageous in terms of foaming. To overcome these commercial grades need to be modified. Therefore, several chemical modifiers were used to induce crosslinking, chain extension or grafting by means of reactive extrusion on a twin-screw extruder. The best results were achieved with organic peroxide. With this modifier the melt strength and the crystallization rate were improved and lead to foams with a closed-cell structure and low density. Organic peroxide was found to be more efficient than the commercial multifunctional epoxide modifier.
Isothermal Cure Rheology and Chemo-Physical Correlation of a Medical Grade, Two-Part Epoxy-Amine Adhesive System
A series of isothermal, rheometric, cure experiments were conducted in small-amplitude oscillatory shear (SAOS) rheology mode at various temperatures of interest in an effort to probe relevant cure rheological behaviors of a two-part, reactive epoxy-amine adhesive system. Based on the measured rheograms, various characteristic cure times and physical transitions were quantitatively identified. A nonisothermal, calorimetric cure experiment was carried out by using a differential scanning calorimeter (DSC). The results were further utilized to analyze the “instant” chemo-physical changes at various partly-cured states of the adhesive system under simulated isothermal or non-isothermal cure conditions by using the StepScan™ DSC method. The chemo-physical correlations between the rheologically-measured physical transitions and calorimetrically-measured chemical changes are established for the purposes of understanding relevant mechanisms that govern isothermal cure processes of the adhesive system and providing practical engineering insights for advance process development in making medical devices.
Linear and Nonlinear Behavior on PP/CNT Composites Prepared by Continuous Ultrasonic Twin-Screw Extrusion
PP/CNT nanocomposites of various concentrations are prepared using ultrasonic aided extrusion without and with ultrasonic treatment to achieve different CNT dispersion levels. The linear and nonlinear rheological behaviors of these nanocomposites are studied using small and large amplitude oscillatory flow (SAOS and LAOS) start-up shear flow and step-strain relaxation. The improved dispersion of CNTs by ultrasonic treatment is found to increase the shear stress level at different shear rates. The relaxation modulus of PP/1wt%CNT composites is found to be lower at low strains, than that at high strains, due to the instability of the filler network. LAOS results of PP and PP/1wt%CNT composites indicate that the elastic and viscous Lissajous curves are ellipses. In contrast, for PP/3wt%CNT and PP/5wt%CNT composites at high strain amplitudes the shape of the Lissajous curves are distorted, as a result of the nonlinearity. The intensity of the third harmonic increases with the strain amplitude and CNT concentration. Ultrasonic treatment of PP/CNT nanocomposites, leading to an improved CNT dispersion, further enhances the nonlinear behavior. At low CNT concentrations, values of G’ and G’’ decrease with the strain amplitude, but at high concentrations a value of G’’ exhibits a maximum with the strain amplitude. Chebyshev polynomials are used to decompose the elastic and viscous stresses. At high strain amplitudes, both the elastic and viscous stresses exhibit a nonlinear behavior. All the PP/CNT composites exhibit a strain-stiffening behavior. The ratio of viscous contribution v3/v1shows a peak with increasing of strain amplitude, meaning that the intra-cycle shear thickening followed by intra-cycle shear-thinning behavior with the strain amplitude. These intra-cycle nonlinear behaviors are increased with the increase of CNT concentration and enhanced by the ultrasonic treatment.
Comparison of Selective Localization of SWNTS in Blends of Powdered PA6/Polypropylene and Granule PA6/Polypropylene
Two different form of polyamide 6 (PA6), granule and powder, was employed to produce the immiscible PA6/polypropylene (PP) blend (50/50 by wt.%) composites filled with prestine single-walled carbon nanotube (SWNTs) contents of 2 wt.%. The effect of different physical form of PA6 on the selective localization of SWNTs was studied by measuring the morphological, rheological properties and thermal conductivity. The images of Scanning Electron Microscopy (SEM) confirmed that SWNTs were selectively located in PA6 phase, which is in good agreement with the results of wettability coefficient calculation. Due to pre-interaction between powdered PA6 and SWNTs, PA6 phase was shown as discontinue-like morphology compared to that of composite using granule PA6. For this reason, the capable volume, where SWNTs is selectively located, and its network is formed, is more confined in the composite, leading the lower storage, loss modulus and complex viscosity at low frequency region. The thermal conductivity of powdered PA6 contained composite had about 10% higher than that of granule PA6 contained composite. This is probably because at the same loading, the effective volume concentration of the tubes in the PA6 phase of composite prepared by powdered PA6 is higher than that of composite prepared by granule PA6.
Low Density HMS Polypropylene Foam: Controlling Foam Density and Cell Morphology
Linear isotactic polypropylene (PP) is used in a vast array of applications because it provides mechanical strength, chemical resistance, and thermal stability. However, semi-crystalline linear PP has limited use in low-density foam applications, which are dominated by amorphous polymers, such as polystyrene. This paper discusses technical challenges that have limited the use of PP in low-density, extruded foams. Specifically, the challenge of controlling foam density along with closed cell percent and cell count is addressed. The rheological properties have been evaluated in terms of viscosity, elasticity and melt strength which show good foaming potential. Interactions between the HMSPP polymer, linear PP blend polymers, blowing agent type, additive formulation, and process variables are investigated here for a new, developmental HMSPP grade. Braskem has developed a proprietary technology to produce High Melt Strength Polypropylene (HMSPP), branded as the Amppleo family, with a specific long chain branching configuration that helps overcome the limitations of linear PP when foaming to low densities of 150-50kg/m3.
Nanocomposites of SEBS/CNT for Electromagnetic Shielding: Effect of Processing Method and Maleic Anhydride
In this work, poly (styrene-b-ethylene-ran-butyleneb- styrene) (SEBS) and SEBS grafted maleic anhydride (SEBS-MA) and carbon nanotube (CNT) nanocomposites (SEBS/CNT and SEBS-MA/CNT) were prepared for electromagnetic shielding applications. Two different melt compounding methods were used, mixing followed by extrusion, and mixing followed by compression molding. In order to assess the morphologies and properties, the different nanocomposites were characterized through rheology, AC electrical conductivity measurements, and electromagnetic shielding analysis. Three different nanocomposites prepared in this work presented the requirements necessary to be used commercially for electromagnetic shielding applications. The higher electrical conductivity, around 6.0E-4 S.cm-1, and the higher electromagnetic shielding effectiveness, 31.67 dB, were achieved by the nanocomposite of SEBS/CNT with 5 wt% of CNT prepared by melt mixing followed by compression molding. This nanocomposite presented an attenuation of 99.93 % of the incident electromagnetic radiation.
An Approach to Improve the Prediction of Injection Pressure in Simulation Technology
Injection molding process simulation is a complex phenomenon wherein a thermoplastic material in melt state is injected into a cavity. The polymer melt replicates the details from the cavity and retains it as it solidifies and subsequently is ejected out of the mold. The Computer Aided Engineering (CAE) tools used for process simulation should be able to estimate the flow pattern, temperature distribution, shrinkage arising from material compressibility, viscous heating, pressure distribution, solidification, crystallization, fiber orientation, clamp force, etc. Despite many complexities arising from both material and molding process behavior, the CAE tools have evolved and matured to be reliable, accurate and useful in providing insightful details that can be used during product and process development. In some situations these tools still lack accuracy and overall reliability while analyzing some of the complex molding processes like thin wall molding, gas-assisted molding, etc. and needs to be studied to reduce these gaps and increase manufacturing predictability. In this report a systematic approach and detailed steps to further improve the overall accuracy of CAE prediction is described. This covers critical aspects like measurement of mold surface temperature, melt temperature and reduce their uncertainty while using them as inputs in CAE. Through a detailed in-mold rheological study the influence of injection speed on pressure to mold the part is studied leading to derivation of molding window. The pressure loss that occurs in the machine screw barrel can be significant and is captured through an air-shot study. All these studies provide insights about the process and forms the basis for setting up the model in CAE, which is more representative of the actual process consisting of the part, material, the flow channels, initial temperature conditions of melt and mold. Using this approach and with the inclusion of improved characterization of resin’s viscosity in the CAE material model, we are able to predict the peak pressure in this representative tool within 10% of actual value for LEXAN™ LS1.
Comparison of the Processing Properties of Thermoplastic Elastomers and Thermoplastics Using Gas Assisted Injection Molding
The process of gas assisted injection molding (GAIM) with thermoplastic materials has been investigated comprehensively in recent years. This has been done using both experimental and numerical (mostly finite-element-method) studies. In the study presented in this paper the possibilities of using thermoplastic elastomers (TPE) within this process are shown and compared to typical thermoplastic polymers. The results include the obtained residual wall thicknesses and measurements of gas front velocities. The minimal wall thicknesses obtained with TPE materials and thermoplastics at high gas pressures within this study are of similar level. The values for lower pressure levels however are significantly influenced by the rheological properties of the particular materials. The distinctive shear thinning of the TPE materials and the Newtonian flow behavior of the thermoplastics used in this study have major influence on the melt displacement velocity and the residual wall thickness.
Rheology of Acrylonitrile Butadiene Styrene with Hollow Glass Microspheres for Extrusion Process
The influence of hollow glass microspheres (HGM) on the rheological properties of a commercially available Acrylonitrile-butadiene-styrene (ABS) polymer was investigated. ABS/HGM composites were prepared with various HGM contents. The rheology of the ABS/HGM composites was characterized to provide insight into the influence of the temperature and sphere concentration on the flow behavior under shear. Linear viscoelastic measurements show that both complex viscosity and storage moduli exhibit about 4 orders of magnitude increase with increasing HGM concentration from 40 vol% to 50 vol%. The viscosity increase is more pronounced at low frequency shear rates.
Computer Aided Output Improvment of a High Capacity Blown Film Extrusion Line
For improving the output of high capacity blown film extrusion lines usually, the limiting factor, namely the air-cooling ring, is substituted or modified. Therefore, the production process has to be interrupted which is time and cost intensive. Primarily the major disadvantage of this experimental strategy is the uncertainty about the outcome. In detail, not all the thermodynamic and fluidic phenomena caused by the changing cooling configuration, and their impact on the formation of the bubble, are predictable in advance. To overcome these problems and to understand all the effects, which take place inside the bubble formation zone a numerical procedure has been developed and validated in previous works [1, 2, 3]. The so-called Process Model is capable of simulating the formation of the bubble with regard to changing cooling configurations and rheological behavior. According to industrial concerns, the modeling procedure was adapted to fulfill the requirements for simulating a high capacity blown film process . In this paper, the first results for the numerical optimization of an industrial high capacity blown film process, using the adapted Process model, will be presented. Furthermore, a developed evaluation strategy for the CFD-results will be used to point out the positive effects of the modified cooling configuration. Based on the simulation results, the experimental validation will prove the applicability of the computer-assisted designing and optimizing strategy. For this purpose, the best virtual outcome will be manufactured and transferred to the current high capacity blown film line. It will be shown that output improvements of approximately 10% are achievable without neglecting the quality of the final film product.
Rheological Characterization of Medical Thermoplastic Polyurethanes
The use of thermoplastic polyurethanes (TPUs) in the medical device industry is widespread due to the unique combination of biological properties, abrasion resistance, and processability that they provide. Phase separation at the microscopic level within the morphology of TPUs results in the presence of hard and soft polymer block segments, creating these desirable characteristics. However, the microphase separation also complicates the understanding of TPU structural properties, particularly their flow properties, and creates difficulties during melt processing. Properties of several TPUs were characterized with a novel rheological method to quantify the effects of time dependence and are reported in this study.
Polymer Modifications in Asphalt Roofing
Polymers have been widely used in asphalt roofing industries in order to reduce premature failure and improve final performance such as cracking and impact resistance, which is difficult to be achieved by asphalts alone. This work focuses on styrene-butadiene-styrene block copolymer (SBS), styrene-ethylene/butylenestyrene copolymer (SEBS), and Elvaloy modifications on asphalt roof coatings. The thermal susceptibility, low temperature cracking propensity were investigated and compared with unmodified version to present the advantages and challenges of polymer modifications. From the perspective of manufactures, the possibility to double stack roof pallets were estimated based on blocking resistance evaluation via an axial rheological method.
Effect of Pre-Cross-Linking on Mechanical and Rheological Properties of Solid Silicone Rubber and its Processability in Extrusion Blow Molding
The blow molding process offers the possibility of reproducible, fully automatic and therefore cost-efficient mass production of complex hollow bodies. Due to the poor mechanical properties of uncured rubber, it has not yet been used for the manufacturing of elastomeric hollow parts. In this contribution, it is shown that with a defined pre-cross-linking of solid silicone rubber the blow molding of the material is possible. With pre-cross-linking the mechanical material properties can be adjusted precisely. This allows parison extrusion without strong drawdown. During the forming, it provides the necessary elasticity while maintaining the weldability and formability of the material. But pre-crosslinking also influences the materials rheological properties. Preliminary investigations showed pre-crosslinking has to take place in the blowing head bevor the material leaves the die. Therefore, changes in rheological material behavior are investigated and considered for the flow channel design. It is shown that the pre-cross-linking allows the blow molding of elastomeric hollow bodies with a surface stretch ratio of 3.6 to 1. However, precross- linking can also lead to flow instabilities such as wall slippage and melt fracture.
Effect of MnO2 Nanowire on Rheological and Dielectric Properties of PVDF/CNT Hybrid Nanocomposites
Poly (vinylidene fluoride) (PVDF) matrix hybrid nanocomposites incorporating MnO2 nanowire (MnO2NW) and Carbon nanotubes (CNT), were fabricated by melt mixing in a batch mixer followed by hot pressing. Dielectric properties of fabricated nanocomposites were studied in X-band frequency (8.2-12.4GHz). The conductive CNT increased the dielectric permittivity of the PVDF by serving as a nanocapacitor. Increasing CNT loading enhanced dielectric loss due to the formation of a conductive network. Adding MnO2NW increased the dielectric permittivity while decreasing dielectric loss. Rheology coupled with dielectric properties and electrical conductivity measurements of the nanocomposites showed the effect of MnO2NW, as secondary nanofillers, on the CNT percolative network. We attribute the superior dielectric properties of the hybrid nanocomposites to the role of MnO2NW on improving the dispersion state of CNT (confirmed by rheology) and also its barrier role on hindering the CNT network formation.
Rheology of Ballistic Clay: the Effect of Temperature and Shear History
Ballistic clay is used as a backing material for standards-based ballistic resistance tests for the purposes of providing a measure of the energy transferred to the body when a threat is defeated. However, this material exhibits complex thermomechanical behavior under actual usage conditions. In this work, we characterize rheological properties of the standard backing clay material, Roma Plastilina No. 1, used for body armor testing, using a rubber process analyzer. Test methods employed include oscillatory strain sweep, frequency sweep, and oscillatory strain ramp. The results show that the material is highly nonlinear, thermorheologically complex, and thixotropic. The modulus decreases under dynamic deformation and partially recovers when the deformation is discontinued. Experimental protocols developed in this study can be applied for the characterization of other synthetic clay systems.
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